fix mir-opt test

Author: nikomatsakis

src/test/mir-opt/basic_assignment.rs

@@ -56,7 +56,7 @@ fn main() {
 //        StorageLive(_4);
 //        _4 = std::option::Option<std::boxed::Box<u32>>::None;
 //        FakeRead(ForLet, _4);
-//        AscribeUserType(_4, o, Canonical { variables: [], value: std::option::Option<std::boxed::Box<u32>> });
+//        AscribeUserType(_4, o, Ty(Canonical { variables: [], value: std::option::Option<std::boxed::Box<u32>> }));
 //        StorageLive(_5);
 //        StorageLive(_6);
 //        _6 = move _4;